Recent diamond films produced by the methods such as chemical vapor deposition (CVD) have been considered technologically as a material for both semiconductor layers and insulating layers having remarkable properties, since diamond being a wide band gap material (forbidden band width is about 5.5 eV) is suitable for an electronic material for various fields because of its properties. For example, diamond can be made a semiconductor by doping, and it has an excellent electron emitting property, hardness, wear resistant property, high thermal conductivity and chemical inertness.
Conventional diamond films are produced from the source gas consisting of a carbon-based gas and a hydrogen gas by the vapor deposition or some other methods. In such a method, films of about at least 0.5 .mu.m thick are formed on substrates such as silicon. The crystal grains' diameter is approximately 0.5 .mu.m or more, and the crystal grain density ranges from about 10.sup.8 -10.sup.9 number/cm.sup.2. For producing these diamond films, the growth nuclei should be controlled in the initial growing process, since in general, very few growth nuclei will be generated when a diamond film is formed on a substrate such as silicon without any treatment, and film-formation will be difficult. In addition, the growth nuclei have relations with the properties of the films and with the adhesion to the substrates. Therefore, a so-called "scratching" has been carried out in conventional arts as a pretreatment for substrates. Specifically, growth nuclei are formed by scratching the surface of the substrate material using diamond abrasive grains having a comparatively large diameter of from several .mu.m to tens of .mu.m. Publication of Japanese Patent Application (Tokkai Hei) No. 5-271939 discloses that a spherical shape diamond powder with an average diameter of 2-50 nm is used for scratching the surface of the substrates. Publication of Japanese Patent Application (Tokkai Hei) No. 5-58784 discloses that diamond growth nuclei are formed by using diamond ultra-fine grains of diameter from 2 to 100 nm on the substrates.
Such diamond films obtained by the conventional techniques are, however, comparatively large in the crystal grain sizes and the continuous films are comparatively thick. As a result, the freedom degree for the thickness of the formed continuous film was relatively low and the films were not sufficiently dense. In addition, the surface flatness and the properties of a diamond film are not homogeneous, and the films are not sufficiently adhered to substrates. Factors of such problems are closely related to the nucleation enhancement that is carried out as a pretreatment. Though the conventional treatments such as scratching or simply applying diamond ultra-fine grains to a substrate will enhance the nucleation, the nucleation density is no more than 10.sup.8 -10.sup.9 per 1 cm.sup.2, and thus, diamond films with sufficient density, adhesion, and homogeneity in a film is difficult to be obtained. A large substrate might not be treated with a sufficient homogeneity. Another problem is the insufficient repeatability of nucleation effect obtained for every batch.